1. Field of the Invention
The present invention relates to a light beam scanner for scanning an object to precisely measure the shape and size of the object, and particularly to a light beam scanner which converges a laser beam into a fine beam spot to scan a two-dimensional plane with the beam spot in an optional direction and measure a fine two-dimensional pattern on the plane.
2. Description of the Related Art
Precision processing technology has improved dramatically in recent years, and in semiconductor and precision machinery industries, is being employed to form two-dimensional fine patterns of the order of micrometers. This results in a need for precise measurement of the size and shape of a fine pattern, and the precise measurement requires high spatial resolution. To improve the spatial resolution, a light beam or laser beam scanner is needed which can converge a laser beam into a fine spot and scan a given range of an object at a fine scanning step of, for example, 0.01 micrometer.
In some cases, the light beam scanner must two-dimensionally scan an area of several tens or several hundreds of square micrometers at a scanning step of, for example, 0.05 micrometer.
The scanner comprises a scanning element for scanning an object with a light beam. The scanning element is usually an acousto-optic deflection element, a galvanomirror, or a polygon mirror. In linearly scanning the object with a light beam, the scanning element is combined with various lenses to form a scanner. In two-dimensionally scanning the object with a light beam, two scanning elements are combined with various lenses to form a scanner. The scanning direction of each of the scanning elements is, for example, the direction of an axis X or Y depending on the optical system of the element.
A conventional two-dimensional scanner for measuring a two-dimensional pattern usually carries out raster scanning on a plane defined by orthogonal axes X and Y.
A linear or a two-dimensional light beam scanner predetermines its scanning direction in the direction of an X or Y axis, or both, and carries out raster scanning in the orthogonal directions. A pattern on a two-dimensional object is not always in parallel with the axes X and Y but may extend in optional directions. The conventional raster scanner cannot measure such a pattern extending in optional directions.
To direct a light beam in an optional direction other than the X-axis or Y-axis direction, it is necessary to modulate driving signals for driving X-axis and Y-axis scanning elements to simultaneously scan the X and Y axes, thereby changing a scanning direction. Alternatively, third and fourth scanning elements must be added to freely change the scanning direction. Modulating the driving signals requires, however, a complicated driving circuit, and adding other scanning elements complicates the scanner optical system. Due to these problems, it is difficult to scan an object in an optional direction. In addition, a range to be scanned must have uniform scan characteristics.
A conventional bar code reader will now be explained. To read information from a bar code, it is necessary to surely scan a bar code label with a laser beam. In the field of physical distribution, the orientation and position of a bar code label attached to an article vary widely. This results in a need for a multidirectional scanner that can stably scan a bar code label with a laser beam irrespective of the orientation and position of the bar code label. A simple bar code scanner frequently used is a linear scanner comprising a scanning element such as a polygon mirror or a galvanomirror which scans only a line with a laser beam. In this case, the bar code label must be positioned at the laser beam scanning position to read information from the label.
To multidirectionally scan the bar code label, it is necessary to two-dimensionally direct a laser beam. FIG. 1(A) shows a simplest form of a two-dimensional bar code scanner.
In FIG. 1(A), a laser source 11 emits a laser beam, which is separated in two directions by a half mirror 12. One of the separated beams, i.e., a beam 230 passed through the half mirror 12 is reflected by a reflector 13 to irradiate a polygon mirror 14. The other of the separated beams, i.e., a beam 235 reflected by the half mirror 12 directly irradiates the polygon mirror 14. The polygon mirror 14 changes the direction of a reflected beam depending on its rotational state. The beam 230 is reflected by the polygon mirror 14 as a reflected beam 240, which is reflected by a reflector 15 to scan an object in a direction X.
On the other hand, the beam 235 is reflected by the polygon mirror 14 as a reflected beam 245, which advances differently from the reflected beam 240. The reflected beam 245 is reflected by a reflector 16 to scan the object in a direction Y. In this way, the object is scanned in the crossing directions with the reflected beams.
According to the method of FIG. 1(A), the reflectors 15 and 16 are inclined in synchronism with the rotation of the polygon mirror 14 to realize two-dimensional crosshatch scanning as shown in FIG. 1(B). This method is applicable when the attaching position of a bar code label varies in the orthogonal two directions.
In FIG. 1(A), various lenses for converging laser beams are omitted.
FIG. 2(A) shows another example of the conventional multidirectional scanner. A laser source 20 emits a laser beam, which is reflected by a reflector 21 to irradiate a rectangular prism 22. The rectangular prism 22 is turned by a motor 23. According to the turning of the rectangular prism 22, the direction of a reflected beam changes. There are "n" reflectors 24a to 24n disposed concentrically around the rectangular prism 22. In response to the turning of the rectangular prism 22, the reflected beam successively irradiates the reflectors 24a to 24n, which provide radial scanning beams as shown in FIG. 2(B). The number of the radial scanning beams is determined by the number of the reflectors disposed around the rectangular prism 22. In FIG. 2(A), various lenses for converging laser beams are omitted.
According to the conventional bar code scanners shown in FIGS. 1(A) and 2(A), the multidirectional scan is realized by reflecting a laser beam in different directions and converting the reflected beams into two-dimensional scan patterns by the use of many reflectors. In these scanners, the number of scanning lines is determined by the number of the reflectors so that, to increase the number of the scanning lines, the number of the reflectors must be increased. This may enlarge the size of the optical system of the scanner and complicate the adjustment of the optical system.